A gas blowing nozzle for blowing an inert gas such as a nitrogen gas or an argon gas, a carbon monoxide gas or a carbon dioxide gas into a molten metal in a refining furnace is generally arranged in a bottom or a side wall of a molten metal refining vessel such as an electric furnace or a converter.
Since the gas blowing nozzle is required to have thermal spalling resistance, abrasion resistance, and corrosion resistance to hot metal, molten steel, slug and the like, the gas blowing nozzle is generally prepared by installing one or a plurality of narrow metal pipes (for example, stainless steel) for blowing a gas in a carbon-containing refractory such as MgO—C brick in the form that the pipes are embedded so as to penetrate through the refractory.
A single pipe type nozzle having a large diameter is used as a gas blowing nozzle used to blow a large amount of a gas, and a narrow pipe type nozzle having a plurality of narrow metal pipes embedded so as to penetrate through a refractory is used in the applications that are not particularly necessary to blow a large amount of a gas and are desirable to blow a gas of minute bubbles.
Furthermore, the gas blow nozzle is that during the use, the temperature of the nozzle itself is increased, and by carburization phenomenon that carbon in a carbon-containing refractory permeates in narrow metal pipes, a melting point of the narrow metal pipes is decreased and the narrow metal pipes melt.
When the narrow metal pipes melt, a molten steel flow caused by the blowing of a gas directly collides against a carbon-containing refractory, and the refractory tends to wear down. For this reason, a life of the gas blowing nozzle itself becomes short.
Therefore, to solve such problems, technologies as in the following Patent Documents 1 to 6 are proposed.
Patent Document 1 proposes a gas blowing nozzle in which narrow metal pipes for gas blowing are coated with a refractory such as a carbon-free castable, and then embedded in a carbon-containing refractory.
However, in the case of the gas blowing nozzle of Patent Document 1, a refractory having poor thermal spalling resistance and corrosion resistance such as a carbon-free castable wears down first, and a castable portion becomes a rate-determining factor of a life. Therefore, there is the problem that service life cannot further be prolonged in the state that a nozzle is maintained in a give length.
Patent Document 2 proposes a plug for blowing an alumina-carbonaceous gas in which a slurry liquid comprising MgO type ultrafine powder as a main component is applied to the outer periphery of a plurality of gas blowing narrow metal pipes provided in an alumina-carbonaceous refractory to form an MgO coating layer.
However, in the case of the gas blowing plug of Patent Document 2, in installing MgO-coated narrow metal pipes in the carbon-containing refractory, the coating layer peels, resulting in carburization in the peeled portion, and sufficient effect is not obtained. Therefore, there is the problem that the service life cannot further be prolonged in the state that a nozzle is maintained in a give length.
Patent Document 3 proposes a gas blowing nozzle in which a fire-resistant sintered body is provided between a carbon-containing refractory and gas blowing narrow metal pipes, thereby preventing carburization.
However, in the case of the gas blowing nozzle of Patent Document 3, it is generally necessary to interpose a mortar between the gas blowing narrow metal pipes and the fire-resistant sintered body, and there is the problem that the mortar having poor abrasion resistance and corrosion resistance wears out first, and damage expands from the mortar portion. Furthermore, even in the case of Patent Document 3, there is the problem that the service life cannot further be prolonged in the state that a nozzle is maintained in a give length.
Patent Document 4 proposes a gas blowing nozzle in which alumina or magnesia is sprayed to gas introducing narrow metal pipes, thereby preventing carburization.
However, in the case of the gas blowing nozzle of Patent Document 4, because a thermal expansion coefficient differs between the gas introducing narrow metal pipes and a spraying material, there is the problem that the spraying material peels due to expansion difference, and the narrow metal pipes are carburized in the peeled portion. Therefore, even in the case of the constitution of Patent Document 4, there is the problem that the service life cannot further be prolonged in the state that a nozzle is maintained in a give length.
Patent Documents 1 to 4 above each intend to improve durability by suppressing carburization, and the service life can be prolonged for only the period that the carburization is suppressed. However, if the service life is intended to further prolong, it is necessary to further lengthen the nozzle.
However, if the nozzle is lengthened until the target service life is obtained, a distance from a gas discharge hole to a surface of a molten steel becomes short, and this gives rise to the problem that refining efficiency is decreased by the deterioration of stirring efficiency.
Furthermore, in the case that the nozzle is excessively projected from a surface of a hearth in a refining furnace, a surface to be heated is increased, and thermal spalling or structural spalling tends to cause.
Furthermore, where the entire hearth in the refining furnace is raised for only the lengthened portion of the nozzle, this gives rise to the problem that not only refractory costs are increased, but a given amount of a molten steel cannot be refined.
Therefore, the actual situation is that it is difficult in the constitutions of Patent Documents 1 to 4 to further prolong the service life by lengthening the nozzle.
By the way, Patent Documents 1 to 4 relate to the technology of intending to increase the number of operation of a refining vessel by improving durability of the nozzle itself becoming a rate-determining factor of a life, and further propose the technology of increasing the number of operation of the refining vessel by a repairing or switching method of a gas blowing nozzle.
For example, Patent Document 5 discloses a bottom blowing converter in which gas blowing nozzles installed on many places are previously embedded in a furnace bottom refractory and distances from the surface of the furnace bottom refractory to the tips of the nozzles are varied respectively.
However, in Patent Document 5, a gas blowing nozzle during standby does not aerate a gas. Therefore, control of residual thickness is difficult, and it is difficult to switch gas blowing with good timing. If messing up the timing, a molten steel penetrates inside of the nozzle, and there is a risk of causing steel leakage. The gas blowing nozzle of Patent Document 5 is not a fine pipe type, but a single type using a pipe having large diameter. Therefore, there is the problem that a risk of steel leakage is further high.
Moreover, there is the problem that construction is complicated and troublesome such that a blind cap brick or a blind cap receiving brick is provided in the furnace-side tips of the nozzle.
Furthermore, there is the problem that opening work of a switching tuyere and occluding work of the used tuyere are troublesome when switching gas blowing.
Patent Document 6 proposes a molten metal refining vessel in which a gas blowing nozzle having gas introduction pipes opened from the time of beginning of use of the molten metal refining vessel, and a gas blowing nozzle in which its tip surface is in contact with a molten metal at the time of beginning of use of the vessel, a gas introduction pipe is occluded up to the tip, and the gas introduction pipe is opened by a quick exchange method are arranged on a bottom or a side wall of the molten metal refining vessel.
Patent Document 6 further proposes an operation method of the molten metal refining vessel in which in refining a molten metal by the molten metal refining vessel, after initially using any two of the gas blowing nozzles, other two gas blowing nozzles are used by the quick exchange method, the initially used nozzles are occluded, and about two nozzles as one pair are alternately used according to the number of use of the refining vessel.
However, in the case of the molten metal refining vessel and the operation method thereof of Patent Document 6, for example, in switching nozzles, a blind brick must de destroyed by drilling or the like. Furthermore, there is the problem that a series of exchange operation is troublesome in that works occur such that an iron shell at the first gas blowing nozzle side is removed, a refractory receiving nozzles is dismantled, and the space is filled with an amorphous refractory, and the efficiency is poor. Furthermore, there are the problems that a gap may be formed in the opening of tuyere and the gas blowing nozzle inserted, a molten metal may penetrate the gap, resulting in steel leakage, and reliability is low.